The invention relates to a semiconductor laser comprising a semiconductor body having a substrate of a first conductivity type and a layer structure provided on it, said layer structure comprising successively at least a first passive layer of the first conductivity type, a second passive layer of the second opposite conductivity type and an active layer located between said first and second passive layers and having a pn junction which at a sufficiently high current strength in the forward direction generates coherent electromagnetic radiation in a strip-shaped active region of the active layer located inside a resonant cavity. The first and second passive layers have a lower refractive index for the generated radiation and a larger band gap than the active layer, and a current-confinement blocking layer is provided which has a strip-shaped interruption at the area of the active region, the second passive layer and the substrate being electrically connected to connection conductors.
A semiconductor laser of the kind described is known from U.S. Pat. No. 3,984,262.
In semiconductor pn lasers, more particularly in lasers of the double heterojunction type (so-called pH lasers), different constructions are in use in order to ensure that the pumping current remains confined to the strip-shaped active part of the laser structure in which the coherent radiation is generated, in order to obtain the desired laser action at the lowest possible threshold current and to prevent overheating.
According to the simplest method, it is ensured that one of the electrodes is in contact only with a strip-shaped part of the laser surface and outside this region is separated from the semiconductor surface by an insulating layer of, for example, silicon oxide. However, in this construction the distance between the electrode and the active layer is fairly large and current spreading occurs over this distance.
In another known method, a high resistance region is formed outside the strip-shaped active region of the laser, this high-resistance region extending from the surface to the proximity of the active layer or, if desired, even through this layer. Such a high resistance region, which effectively confines the current to the active part of the semiconductor laser, is generally formed by a proton bombardment, which disturbs the crystal structure and hence considerably increases the electrical resistance. This method is described in U.S. Pat. No. 3,824,133. However, this method has the disadvantage that expensive equipment is required for carrying out such a proton bombardment and that this bombardment is a fairly laborious operation.
In order to obviate the above disadvantages, a third method has been developed, which is described in the aforementioned U.S. Pat. No. 3,984,262 and consists of including in the crystal structure in the proximity of the active layer a buried blocking layer, which may consist of high resistance material or of semiconductor material of such a conductivity type that the blocking layer forms a pn junction with the adjoining semiconductor material. In the operating condition this pn junction may be biased in the reverse direction or may be biased in the forward direction in a manner such that no or substantially no current flows through the pn junction. Such a blocking layer can be formed in a simple manner, for example by diffusion or ion implantation or by epitaxial growth, and provides an excellent current confinement. Lasers of this construction, especially the more advanced versions thereof, such as the DCPBH (Double Channel Planar Buried Hetero) laser described inter alia in Electronics Letters, Oct. 28, 1982, Volume 18, No. 22, p. 953-954, are therefore particularly suitable for many applications.
In important fields of application, such as optical communication, it is required to cause the laser to operate at a very high modulation frequency, for example at 1 GHz or higher. Although the aforementioned lasers having a current-confining blocking layer have proved to operate satisfactorily in other respects, at these high frequencies problems arise which become manifest in a serious limitation of the modulation bandwidth.